One technology for after-treatment of engine exhaust utilizes selective catalytic reduction (SCR) to enable certain chemical reactions to occur between NOx in the exhaust and ammonia (NH3). NH3 is introduced into an engine exhaust system upstream of an SCR catalyst by injecting urea into an exhaust pathway, or is generated in an upstream catalyst. The urea entropically decomposes to NH3 under high temperature conditions. The SCR facilitates the reaction between NH3 and NOx to convert NOx into nitrogen (N2) and water (H2O). However, as recognized by the inventor herein, issues may arise upon injecting urea into the exhaust pathway. In one example, urea may be poorly mixed into the exhaust flow (e.g., a first portion of exhaust flow has a higher concentration of urea than a second portion of exhaust flow) which may lead to poor coating of the SCR and poor reactivity between emissions (e.g., NOx) and the SCR. Additionally, overly mixing and agitating the urea in the exhaust can likewise cause issues, such as increased deposits.
Attempts to address insufficient mixing include introducing a mixing device downstream of a urea injector and upstream of the SCR such that the exhaust flow may be more homogenous. Other attempts to address urea mixing include a stationary mixing apparatus. One example approach is shown by Cho et al. in U.S. 2013/0104531. Therein, a static mixer is located in an exhaust passage downstream of an auxiliary tube for injecting urea. The exhaust gas flows through the exhaust passage and combines with a urea injection before flowing through the static mixer.
However, the inventors herein have recognized potential issues with such systems. As one example, the static mixer described above presents limited mixing capabilities due to a directionality of exhaust outflow through the mixer unable to fully mix a laminar exhaust flow. The static mixer inside the exhaust passage also presents manufacturing and packaging constraints. Varying exhaust passage geometries demand an alteration in the manufacturing of the static mixer for the mixer to tightly fit within the exhaust passage. Additionally, the static mixer of Cho does not provide a passage outside of a main exhaust passage for mixing a portion of exhaust gas with the urea injection. As such, the static mixer of Cho may overly agitate the urea injection, which may lead to urea deposits and poor coating of the SCR.
In one example, the issues described above may be addressed by a mixer comprising a concave plate located inside an exhaust passage with an opening fluidly coupling the exhaust passage to an auxiliary passage having a urea injector, and where the auxiliary passage is fluidly coupled to a hollow ring physically coupled to an outer surface of the exhaust passage, and where the hollow ring is upstream of the concave plate relative to a direction of incoming engine exhaust gas flow. In this way, a distance of exhaust gas flow is increased compared to an exhaust pipe without the mixer such that mixing is further increased.
As one example, exhaust gas is received by an opening located along a smallest diameter of the concave plate and conducted into a first tube of the auxiliary passage. The auxiliary passage conducts the exhaust gas from the first tube, located in the exhaust passage, to a second tube of the auxiliary passage with a first portion located inside the exhaust pipe and a second portion located outside the exhaust pipe. The second tube further comprises the urea injector located at an intersection between the second tube and a third tube outside of the exhaust pipe. The third tube is configured to flow the exhaust gas and urea mixture to the hollow ring, where the mixture may flow to an annular chamber, located uninterruptedly around an outside of the exhaust pipe. The mixture may flow into the exhaust passage via a plurality of perforations located along a portion of the exhaust pipe corresponding to a location of the hollow ring. Once the mixture is in the exhaust passage, it may flow toward the concave plate, where the mixture may flow through either the opening or through a plurality of perforations leading to an SCR device. In this way, urea dispersion in the exhaust passage may be increased, thereby improving an overall reduction of the SCR device.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.